The invention relates to an apparatus for connecting two workpieces together by welding, in which the workpieces lie on supporting surfaces, are butted together at their edges to form a butt joint, and are subjected to pressure on the opposite side to the supporting surfaces.
Various welding processes are known for joining two workpieces together, particularly for the fabrication of body components for motor vehicles. Especially worthy of note is laser welding in which the sheet-metal parts are butt-welded to form compound panels.
In the nature of things, the edges of the sheets which are to be welded together are not absolutely straight, and have a certain amount of waviness. This means that the butt joint is closed only where the wavy edges are in contact; at other points the edges stand back from one another, leaving the joint open. According to the industry standard in Europe for laser welding, the gap should not exceed a width of 0.08 mm at any point. This very small tolerance makes it necessary to prepare the edges of the sheets for welding. This is usually done by trimming by laser beam. However, it may also be done by a process according to EP-A 0565846, in which the sheets are butted together and at least one edge is then plastically deformed against the other, closing the gap so that its maximum width no longer exceeds 0.08 mm.
A welding operation usually has the following cycle:
the workpieces are butted together in pairs, without prepared edges, positioned without significant sideways pressure, and fixed by means of clamp elements. At this point the workpieces are first subjected to force. The workpieces may then be processed, for example they may be dressed as described in EP-A 0565846 to bring the gap within the permitted tolerance. This processing causes a second application of force to the positioning system, in which considerable lateral forces can occur. Finally, further, mainly sideways-acting force effects occur as a result of thermal stresses in the laser welding process.
These forces are capable of affecting the joint gap, particularly if the tolerances in the machine allow relative movement of the sheets to be welded, for example if the sheets positioned in pairs are fixed on a clamping table or shuttle. On a shuttle there may be clamped several pairs of workpieces to be joined together. When the first pair of workpieces are processed, the force effect also acts on the other workpiece-pairs.
Logically, the welding machine should be constructed in such a way that the tolerances in the machine are smaller than usual; also, the structure should be made stronger so that the forces generated are no longer capable of altering the relative position of the sheets to be welded.
However, this obvious-seeming solution to the problem results in an extremely costly construction.